Internet Engineering Task Force (IETF) R. Asati
Request for Comments: 5918 Cisco Systems
Category: Standards Track I. Minei
ISSN: 2070-1721 Juniper Networks
B. Thomas
August 2010
Label Distribution Protocol (LDP) 'Typed Wildcard'Forward Equivalence Class (FEC)
Abstract
The Label Distribution Protocol (LDP) specification for the Wildcard
Forward Equivalence Class (FEC) element has several limitations.
This document addresses those limitations by defining a Typed
Wildcard FEC Element and associated procedures. In addition, it
defines a new LDP capability to address backward compatibility.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5918.
Asati, et al. Standards Track [Page 1]

RFC 5918 LDP 'Typed Wildcard' FEC August 2010
Copyright Notice
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Table of Contents
1. Introduction ....................................................32. Specification Language ..........................................43. The Typed Wildcard FEC Element ..................................44. Procedures for the Typed Wildcard FEC Element ...................55. Typed Wildcard FEC Capability ...................................66. Typed Wildcard FEC Element for Prefix FEC Element ...............77. Typed Wildcard FEC Element for Host and Wildcard FEC Elements ...88. IANA Considerations .............................................89. Security Considerations .........................................810. Acknowledgments ................................................911. References .....................................................911.1. Normative References ......................................911.2. Informative References ....................................9Asati, et al. Standards Track [Page 2]

RFC 5918 LDP 'Typed Wildcard' FEC August 20101. Introduction
LDP [RFC5036] distributes labels for Forwarding Equivalence Classes
(FECs). LDP uses FEC TLVs in LDP messages to specify FECs. An LDP
FEC TLV includes one or more FEC elements. A FEC element includes a
FEC type and an optional type-dependent value.
RFC 5036 specifies two FEC types (Prefix and Wildcard), and other
documents specify additional FEC types; e.g., see [RFC4447] and
[MLDP].
As specified by RFC 5036, the Wildcard FEC Element refers to all FECs
relative to an optional constraint. The only constraint RFC 5036
specifies is one that limits the scope of the Wildcard FEC Element to
"all FECs bound to a given label".
The RFC 5036 specification of the Wildcard FEC Element has the
following deficiencies that limit its utility:
1) The Wildcard FEC Element is untyped. There are situations where
it would be useful to be able to refer to all FECs of a given type
(as another constraint).
2) Use of the Wildcard FEC Element is limited to Label Withdraw and
Label Release messages only. There are situations where it would
be useful to have a Wildcard FEC Element, with type constraint, in
Label Request messages.
This document:
- addresses the above limitations by defining a Typed Wildcard FEC
Element and procedures for its use.
- specifies use of the LDP capability mechanism [RFC5561] at
session establishment time for informing a peer that an LDP
speaker is capable of handling the Typed Wildcard FEC.
- specifies use of the Typed Wildcard FEC Element in a Label
Request message.
- specifies the Typed Wildcard FEC Element for the Prefix FEC
Element specified by RFC 5036.
Note that this document does not change procedures specified for the
LDP Wildcard FEC Element by RFC 5036.
Asati, et al. Standards Track [Page 3]

RFC 5918 LDP 'Typed Wildcard' FEC August 2010
Any (future) document specifying a new FEC Element Type (not
defined in RFC 5036) should prescribe whether typed
wildcarding is needed for that FEC Element Type.
Len FEC Type Info: One octet that specifies the length in octets
of the FEC Type Specific information field. It MUST be set to
0 if there is no Additional FEC Type-specific Information.
Additional FEC Type-specific Information (Optional): Additional
information that is specific to the FEC Element Type and that
is required to fully specify the Typed Wildcard. If this field
is absent, then all FECs of the specified FEC Type would be
matched.
Any (future) document specifying Typed wildcarding for any
FEC Element Type should also specify the length and format
of Additional FEC Type Specific Information, if included.
This document specifies one FEC Element Type instance (e.g., Prefix
FEC) for the 'Typed Wildcard FEC Element' in Section 6.
4. Procedures for the Typed Wildcard FEC Element
When a FEC TLV contains a Typed Wildcard FEC Element, the Typed
Wildcard FEC Element MUST be the only FEC Element in the TLV. If an
LDP speaker receives a FEC TLV containing a Typed Wildcard FEC
Element and any other FEC elements, then the LDP speaker should
ignore the other FEC elements and continue processing as if the
message only contains the Typed Wildcard FEC Element.
An LDP implementation that supports the Typed Wildcard FEC Element
MUST support its use in Label Request, Label Withdraw, and Label
Release messages.
An LDP implementation that supports the Typed Wildcard FEC Element
MUST support it for every FEC Element Type defined in [RFC5036].
Receipt of a Label Request message with a FEC TLV containing a Typed
Wildcard FEC Element is interpreted as a request to send one or more
Label Mappings for all FECs of the type specified by the FEC Element
Type field in the Typed Wildcard FEC Element encoding.
An LDP implementation that supports the Typed Wildcard FEC Element
MUST support the following constraints whenever a Typed Wildcard FEC
appears in a Label Withdraw or Label Release message:
Asati, et al. Standards Track [Page 5]

RFC 5918 LDP 'Typed Wildcard' FEC August 2010
1) If the message carries an optional Label TLV, the Typed Wildcard
FEC Element refers to all FECs of the specified FEC type bound to
the specified label.
2) If the message has no Label TLV, the Typed Wildcard FEC Element
refers to all FECs of the specified FEC type.
Backwards compatibility with a router not supporting the Typed
Wildcard FEC element is ensured by the FEC procedures defined in RFC5036. Quoting from RFC 5036:
If it [an LSR] encounters a FEC Element type it cannot decode, it
SHOULD stop decoding the FEC TLV, abort processing the message
containing the TLV, and send an "Unknown FEC" Notification message
to its LDP peer signaling an error.
A router receiving a FEC TLV containing a Typed Wildcard FEC element
for either a FEC Element Type that it doesn't support or for a FEC
Element Type that doesn't support the use of wildcarding, MUST stop
decoding the FEC TLV, abort processing the message containing the
TLV, and send an "Unknown FEC" Notification message to its LDP peer
to signal an error.
A router receiving a FEC TLV containing a Typed Wildcard FEC element
MAY also leverage mechanisms defined in [RFC5919] (say, if it had
zero label binding for the requested FEC type, etc.).
5. Typed Wildcard FEC Capability
As noted above, RFC 5036 FEC procedures provide for backward
compatibility with an LSR not supporting the Typed Wildcard FEC
Element. However, they don't provide means for an LSR that wishes to
use the Typed Wildcard FEC Element to determine whether a peer
supports it other than to send a message that uses the FEC Element
and to wait and see how the peer responds.
An LDP speaker that supports the Typed Wildcard FEC Element MUST
inform its peers of the support by including a Typed Wildcard FEC
Element Capability Parameter [RFC5561] in its Initialization messages
only.
The Capability Parameter for the Typed Wildcard FEC capability is a
TLV with the following format:
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RFC 5918 LDP 'Typed Wildcard' FEC August 2010
The procedures described in Section 4 apply to the Prefix FEC
Wildcard processing.
7. Typed Wildcard FEC Element for Host and Wildcard FEC Elements
There is no need to specify Typed Wildcard FEC Elements for the Host
FEC Element specified by [RFC3036], nor for the Wildcard FEC Element
specified by RFC 5036. The [RFC3036] Host FEC Element has been
removed from RFC 5036, and the Wildcard FEC Element is untyped by
definition.
In other words, the 'FEC Element Type' field in 'Typed Wildcard FEC
Element' MUST NOT be either 0x01 or 0x03.
8. IANA Considerations
This document introduces a new LDP FEC Element Type and a new LDP
Capability, both of which have been assigned by IANA.
IANA has assigned a 'Typed Wildcard FEC Element' code point (0x05)
from the LDP FEC Type Name Space. [RFC5036] partitions the FEC
Type Name Space into 3 regions: IETF Consensus region, First Come
First Served region, and Private Use region. The code point 0x05
is from the IETF Consensus range.
IANA has assigned a 'Typed Wildcard FEC Capability' code point
(0x050B) from the TLV Type name space. [RFC5036] partitions the
TLV TYPE name space into 3 regions: IETF Consensus region, Vendor
Private Use region, and Experimental Use region. The code point
0x050B is from the IETF Consensus range.
9. Security Considerations
No security considerations beyond those that apply to the base LDP
specification [RFC5036] and that are further described in [RFC5920]
apply to use of the Typed Wildcard FEC Elements as described in this
document.
One could deduce that the security exposure is reduced by this
document, since an LDP speaker using the Typed Wildcard FEC Element
could use a single message to request, withdraw, or release all the
label mappings of a particular type (a particular Address Family
Identifier (AFI), for example), whereas an LDP speaker using the
Wildcard FEC Element, as defined in the base LDP specification
[RFC5036], could use a single message to request, withdraw, or
release all the label mappings of all types (all AFIs, for example).
Asati, et al. Standards Track [Page 8]